Heat sealable and easy-opening films are employed on a large scale for temporarily closing containers that include, for example, food products. During use, a consumer tears away the peelable film. To gain consumer acceptance, a number of characteristics associated with a heat sealable and peelable film are desired.
Heat sealable films must be capable of being sealed upon the application of heat. During typical sealing processes, the backing or web layer of the film comes into direct contact with a heated surface such as a sealing jaw. Heat is thus transferred through the backing layer of the film to melt and fuse the inner sealant layer to form a seal. Accordingly the backing layer generally has a higher melting temperature than the inner sealant layer so that the backing layer of the film does not substantially melt and therefore does not stick to the heated surface.
Moreover, if the package to be sealed is designed to contain food, particularly unrefrigerated food, then in order for the product to have an acceptable shelf life (for example at least six months) the seal should be capable of surviving a retort operation. A typical retort process subjects the sealed package to a temperature of 212° F. to 275° F. for 20 to 60 minutes or even up to 100 minutes, depending on the size of the container. During the retort process, gases are generated within the package and pressure increases greatly. Although the retort system may include an over pressure to help balance the package internal pressures, the net result will still be a pressurized package during retorting. Thus, the films used to seal the container must be sufficiently strong to withstand the increased internal pressure and the elevated temperatures.
Because of the need to withstand such pressures, seals used in retort applications are typically difficult to open at room temperature using average manual force. It would be desirable to have a heat sealable film which could withstand the conditions of retort applications yet still be easily opened manually by a consumer. The force required to pull a seal apart is called “seal strength” or “heat seal strength” which can be measured in accordance with ASTM F88-94. The desired seal strength varies according to specific end user applications. For flexible packaging applications, such as cereal liners, snack food packages, cracker tubes and cake mix liners, the seal strength desired is generally in the range of about 1-9 pounds per inch. For example, for easy-open cereal box liners, a seal strength in the range of about 2-3 pounds per inch is commonly specified, although specific targets vary according to individual manufactures requirements. In addition to flexible packaging application, a sealable and peelable film can also be used in rigid package applications, such as lids for convenience items (e.g., snack food such as puddings) and medical devices. Typical rigid packages have a seal strength of about 1-5 pounds per inch. The seal layer can be on the lid or on the container or both.
Another desired property for the heat-sealable films is adequate hot tack. After the film is removed from contact with the heated surface and/or the retort process, the film is cooled to room temperature. Before the inner sealant layer is cooled to room temperature, it should be able to maintain its seal integrity. The ability of an adhesive or sealant layer to resist creep of the seal while it is still in a warm or molten state is generally referred to as “hot tack.” To form a good seal, the hot tack of the sealable and peelable film should be adequate.
It is also desirable to have a low heat seal initiation temperature which helps to ensure fast packaging line speeds and a broad sealing window which could accommodate variability in process conditions, such as pressure and temperature. A broad sealing window also enables high speed packaging of heat sensitive products, as well as, provides a degree of forgiveness for changes in packaging or filling speeds.
Additional desired characteristics for heat sealable films include a low coefficient of friction and good abuse resistance. A low coefficient of friction ensures that the sealant layer can be processed smoothly and efficiently on fabrication and packaging equipment and is particularly important for vertical form-fill-and-seal packaging. Good abuse resistance and toughness is desired, for example, in cereal box liners to withstand tears and punctures from irregularly-shaped, rigid cereals. Additional characteristics include taste and odor performance and barrier or transmission properties.
It has been discovered that certain films allow one or more of the desired goals to be met. Such films are characterized by having a peelable seal layer comprising from about 50 to about 85 percent by weight of a first polymer and from 15 to 50 percent of a second polymer. The first polymer is a propylene based polymer characterized by having a melting point of at least 125° C. together with a Comonomer Composition Distribution Breadth (“CCDB”) less than 2. The second polymer is characterized by having an interfacial adhesion with the first polymer of less than 1 lb/inch.